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In Er3+/Yb3+ codoped Na2O-ZnO-PbO-GeO2-TeO2 (NZPGT) glass fiber, a clear and compact green upconversion amplified spontaneous emission (ASE) trace is observed, and the NZPGT glasses are proved to be a desirable candidate in fabricating low-phonon energy fiber. Intense green upconversion luminescence of Er3+, balanced green and red upconversion emissions of Ho3+, and dominant three-photon blue upconversion fluorescence of Tm3+ have been represented. By varying the excitation power of 974nm wavelength laser diode, a series of green and white fluorescences have been achieved in Tm3+/Er3+/Yb3+ and Tm3+/Ho3+/Yb3+ triply doped glass systems, respectively. These results reveal that high-intensity blue, green, and white upconversion ASE fluorescences, which can be adopted for lighting in minimally invasive photodynamic therapy and minimally invasive surgery, are reasonable to be expected in rare-earth doped NZPGT glass fibers.
©2010 Optical Society of America
1. Introduction
Minimally invasive photodynamic therapy (PDT) is a promising modality to combat cancer due to its less invasive, comparable clinical outcome, reduced side effects, and relatively short healing times [1,2]. In PDT, suitable color-light sources are adopted to activate various photosensitizeres, such as hematoporphyrin derivative (HPD), which assembles into cancer tissues preferentially after systemic controlling. Ultraviolet, blue, and green lights are used for diagnosis and localization of cancer. When cancer cells are irradiated under these radiations 48-72 hours later than intravenous injection of HPD, the cancer cells in the irradiated area will produce red fluorescence exposing their locations. Sequentially, red light irradiating can be carried out on the cancer cells and results in cell death and tissue necrosis directly.
In early years, incandescent lamp, sunlight, and xenon lamp are used for light sources in PDT after filtering out unnecessary wavelengths. However, lower light intensity limits their developments and applications, and so laser light with perfect directivity and strong intensity becomes the popular light source in recent years [3]. The only drawback is that it may accidentally injure the good cells owning to the huge power density, and this disadvantage should be taken seriously. Upconversion amplified spontaneous emission (ASE) fluorescence produced in rare-earth (RE) ions doped glass fibers provides adequate intensity and good directivity and has the potential to be a substitute for optical fiber lighting in PDT. The color and intensity of the upconversion ASE fluorescence can be adjusted by RE ion species and concentration, respectively. As luminescence centers, Er3+ and Ho3+ exhibit attractive green emissions while Tm3+ presents intense blue emission [4–22]. Moreover, multicolor and white light emissions can be generated via the mixed effect of Er3+ (Ho3+) and Tm3+ ions with Yb3+ assistance [23–30]. In minimally invasive surgery, illumination light source plays an essential role, and high-brightness white light source can provide favorable surroundings and improve the operative quality. White upconversion ASE fluorescence in RE ions doped fibers can satisfy the above requirements and the corresponding fibers will become the candidate light sources for minimally invasive surgery.
In this work, lower phonon energy heavy metal germanium tellurite (NZPGT) glasses with high chemical durability and temperature stability are fabricated and adopted as a material actor for special optical fibers. Under the excitation of a 974nm laser diode, the fluorescence characteristics of Er3+/Yb3+, Ho3+/Yb3+, and Tm3+/Yb3+ co-doped NZPGT glasses were investigated. The color coordinates of the mixed RGB upconversion emissions in Tm3+/Er3+/Yb3+ and Tm3+/Ho3+/Yb3+ triply doped NZPGT glass systems have been calculated and marked in the CIE-1931 standard chromaticity diagram, and the color variation tendency is revealed.
2. Experiments
Er3+/Yb3+ co-doped, Ho3+/Yb3+ co-doped, Tm3+/Yb3+ co-doped, Tm3+/Er3+/Yb3+ triply doped and Tm3+/Ho3+/Yb3+ triply doped NZPGT glasses were prepared from high-purity Na2CO3, ZnO, PbO, GeO2, TeO2, Er2O3, Ho2O3, Tm2O3, and Yb2O3 powders. The molar composition of NZPGT host glass is 14 Na2O, 10 ZnO, 7 PbO, 19 GeO2, and 50 TeO2. Additional 0.4 wt% Er2O3 + 1.6 wt% Yb2O3 (sample I), 0.4 wt% Ho2O3 + 1.6 wt% Yb2O3 (sample II), 0.4 wt% Tm2O3 + 1.6 wt% Yb2O3 (sample III), 0.4 wt% Tm2O3 + 0.4 wt% Er2O3 + 1.6 wt% Yb2O3 (sample IV), and 0.4 wt% Tm2O3 + 0.4 wt% Ho2O3 + 1.6 wt% Yb2O3 (sample V) were added respectively based on the host weight. The well-mixed raw materials were firstly preheated in a Pt crucible at 270°C for 3 h, then melted at 880°C for 30 min, and finally quenched in an aluminum mold. The glasses were subsequently annealed at 270°C for 3 h, and cooled down slowly to room temperature.
For optical measurements, the annealed glass samples were sliced into pieces and polished with two parallel sides. Raman spectrum of NZPGT host glass was recorded by a Perkin-Elmer spectrum 2000 NIR FT- Raman spectrophotometer. Visible upconversion emissions were measured using a Perkin-Elmer LS 55 luminescence spectrometer with a R928 photomultiplier detector, and a 974nm wavelength fiber-pigtailed multi-mode diode laser was adopted as the pump source. The color coordinates and correlated color temperature (CCT) were calculated following the method introduced in Ref. 26. The luminescence pictures were taken using a Sony α 200 digital camera.
3. Results and discussion
3.1 Green upconversion emission of Er3+ and Ho3+
A glass fiber based on NZPGT glass system was fabricated using the rod-in-tube method successfully, and a clear, bright, and compact green transmission trace was observed, as shown in the inserted photo (a) of Fig. 1 , exhibiting the NZPGT glasses with high chemical durability and good thermal stability are promising substrates for optical fibers. The strong green upconversion ASE fluorescence observed in the fiber has potential applications for diagnosis and localization of cancer in PDT. In order to understand the fluorescence characteristic deeply, further investigations on NZPGT glass materials were carried out. The upconversion emission spectra of sample I are shown in Fig. 1, and two emission bands peaked at 549 and 668nm are assigned to the (4S3/2, 2H11/2) → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. In this work, no saturation effects were observed in the upconversion fluorescences of Er3+ and following Tm3+ and Ho3+ due to limited excitation power [31]. As depicted in the right insert of Fig. 1, the fitted slopes for the two emissions are 2.07 and 1.84, respectively, indicating that both the two emissions are due to a two-photon excitation process. Under all pump conditions the green emission is consistently the strongest one, and its intensity is almost seven times higher than the red one, resulting in a bright pure green color, as shown in the inserted photo (b) of Fig. 1. The upconversion emission intensity is relevant to the phonon energy of glasses, and then the Raman spectrum of the undoped cladding glass was measured and presented in Fig. 2 , in which the maximum phonon energy is proved to be 793 cm−1. The medium value is lower than those of silicate, phosphate, and borate glasses, which is beneficial to reducing the non-radiative relaxation probability and obtain efficient upconversion efficiency. For red fluorescence of Er3+, the population on 4F9/2 level is limited because of the inefficient nonradiative relaxation from (4S3/2, 2H11/2) level, resulting in a relatively weak emission.
Fig. 1 Upconversion emission spectra of sample I (974nm laser pump powers for curves 1, 2, 3, and 4 are 367, 504, 641, and 845mW, respectively). Right inset: dependence of upconversion emission intensity on excitation power. Inserted photo: (a) photograph of upconversion ASE fluorescence generation and transmission in Er3+/Yb3+ co-doped NZPGT glass fiber when 457mW 980nm single-mode laser was launched into the fiber end; (b) green fluorescence from sample I under 778mW 974nm laser excitation.
Fig. 2 Raman spectrum (solid line) and Gaussian-resolved results (seven dotted lines) of NZPGT host glass.
Figure 3 shows the upconversion emission spectra of Ho3+/Yb3+ codoped NZPGT glasses, and the three emission bands at 548, 659, and 755nm correspond to the (5F4, 5S2) → 5I8, 5F5 → 5I8, and (5F4, 5S2) → 5I7 transitions, respectively. The relationship between emission intensity and excitation power is presented in the right insert of Fig. 3. The fitted slopes for the 548, 659, and 755nm emissions are 1.99, 2.11, and 2.04, respectively, indicating that the three emissions follow a two-photon excitation law. In Ho3+/Yb3+ codoped glass systems such as fluoroindate [32], alkali bismuth gallate [33], and fluorozircoaluminate glasses [34], the green upconversion emission plays a dominant role in the luminescence color, and all the colors are observed to be next to pure green. However, in the case of NZPGT glasses the red fluorescence intensity is comparable with the green one, leading to a yellowish-green color for human eyes, as shown in the inserted photo of Fig. 3. The yellowish-green fluorescence can be combined with blue one to reproduce white light, and if optical fibers exhibiting white upconversion ASE fluorescence are achieved, they will become attractive illumination light sources for minimally invasive surgery.
Fig. 3 Upconversion emission spectra of sample II (974nm laser pump powers for curves 1, 2, and 3 are 236, 504, and 813mW, respectively). Right inset: dependence of up conversion emission intensity on excitation power. Inserted photo: yellowish-green fluorescence from sample II under 236mW pump power.
To understand the upconversion mechanisms, the energy level diagram of Ho3+ and Yb3+ with possible energy transfer (ET) and excited state absorption (ESA) processes are shown in Fig. 4 . Ho3+ ions transfer from the ground state 5I8 to the 5I6 level, and then continue upward from the 5I6 state to the (5F4, 5S2) level, with the Yb3+ ions assistance. After that, the excited Ho3+ ions relax radiatively to the ground state and 5I7 state, producing the 548 and 755nm fluorescences, respectively. For the 659nm emission, there are two possible energy upconversion mechanisms. It can be obtained via populating the 5F5 level by nonradiative relaxation from the (5F4, 5S2) excited state. Also Ho3+ can be excited from its ground state 5I8 to the 5I6 state, relax nonradiatively to the 5I7 level, and then populate the 5F5 excited state by ESA process before finally emitting the red light. Thus, the non-radiative relaxation process is indispensable for the 659nm red fluorescence, and the medium-low phonon energy of NAPGT glasses, which is still higher than those of gallate and non-oxide glasses [33,35], can provide adequate nonradiative relax rates to support the 659nm emission.
Fig. 4 Energy level diagram of Tm3+, Ho3+ and Yb3+ with possible ET and ESA processes in NZPGT glass system under 974nm diode laser excitation.
3.2 Blue upconversion emission of Tm3+
Besides green light, the blue one is also an efficient light source for diagnosis and localization of cancer, and as an upconversion emitting center Tm3+ ion has been widely investigated for its intense blue emission. The upconversion emission spectra of Tm3+/Yb3+ codoped NZPGT glasses are shown in Fig. 5 . The 478nm blue, 649nm red, and 808nm NIR upconversion emission bands correspond to the 1G4 → 3H6, 1G4 → 3F4, and 3H4 → 3H6 transitions, respectively. The fitted slopes of the log-log plots for the dependence of emission intensity on pump power are also shown, and the values are 2.95, 2.86, and 1.96, respectively, indicating that the blue and red emissions are due to a three-photon excitation process, and the NIR one is due to a two-photon excitation process. Although the NIR emission is the strongest one, it does not contribute to the naked-eye vision. In all pump conditions the blue emission intensity is stronger than the red one, playing the dominant role in high- and low-power pumping. Thus sample III exhibits bright blue fluorescence, as shown in the inserted photo of Fig. 5.
Fig. 5 Upconversion emission spectra of sample III (974nm laser pump powers for curve 1, 2, and 3 are 236, 504, and 845mW, respectively). Middle inset: dependence of upconversion emission intensity on excited power. Inserted photo: blue fluorescence from sample III under 236mW pump power.
When pumped by a 974nm diode laser, Tm3+ ions cannot absorb the excitation energy directly due to the lack of matched energy level, but Yb3+ ions can absorb the NIR radiation efficiently and transfer the excitation energy to Tm3+ ions. Three possible excitation processes of Tm3+ were presented in Fig. 4. Firstly, Tm3+ ions are excited to the 3H5 level by ET from Yb3+ and partly relax to the 3F4 level. Secondly, a part of them arrive at the (3F2, 3F3) level by ESA, and then relax to the lower metastable state 3H4. Radiative transition from the 3H4 level to the ground state 3H6 produces the two-photon excited 808nm NIR emission. Thirdly, parts of ions at the 3H4 level are further excited to the 1G4 level by ESA. The excited ions at the 1G4 level then relax radiatively to the ground state 3H6 and the 3F4 level, emitting the 478 and 649nm lights, respectively.
3.3 Multicolor upconversion emissions in RE ions triply doped systems
According to the discussed above, Tm3+/Er3+/Yb3+ and Tm3+/Ho3+/Yb3+ triply dopings were adopted to achieve white light generation by means of varying the intensity ratios among the green, blue, and red emissions. The upconversion emission spectra of Tm3+/Er3+/Yb3+ triply doped NZPGT glasses (sample IV) are shown in Fig. 6 . Four emission bands locating at 476, 549, 667, and 808nm were observed, and the green upconversion emission intensity is more stronger than the other three ones. When sample IV was excited by a 974nm laser diode, most of the ion population distributes on the 4I11/2 level of Er3+ by ET from the 2F5/2 level of Yb3+, and only small parts hold on the 3H5 level of Tm3+. This is because the level difference between 2F5/2 and 4I11/2 is obviously smaller than the value between 2F5/2 and 3H5. Therefore, the green one is always dominant whether the pump power is low or high, resulting in a green color background, and various green colors were obtained in sample IV, as shown in Fig. 7 . Under different pump powers, the CIE-1931 color coordinates (x, y) are calculated and presented in Fig. 8 . All the color coordinates are located in the green area, and with the pump power increasing, it moves along the down left direction towards the cyan area, but maintaining the green color as the dominant one. Compared with the upconversion fluorescence, the upconversion ASE fluorescences in optical fibers exhibits better direction and intensity. Therefore, fibers prepared by sample IV will provide high-brightness green and laurel-green emissions, which can be efficiently used for diagnosis and localization of cancer cells in PDT.
Fig. 6 Upconversion emission spectra of sample IV normalized by the green 531nm wavelength peak intensity under 1042mW 974nm laser excitation.
Fig. 7 Luminescence photos (a), (b), (c), and (d) present the fluorescences of sample IV under 103, 367, 778, and 1024mW pump powers, respectively.
Fig. 8 CIE (x, y) chromaticity diagram indicating the color coordinates of the multicolor upconversion fluorescences in sample IV (■) and sample V (◄). In sample IV, the pump powers corresponding to the points from right to left are 103, 367, 713, and 1042mW, respectively. In sample V, the pump powers corresponding to the points from right to left are 37, 103, 171, 236, and 367mW, respectively.
On the other hand, for white light generation, such stronger green light should be balanced. Therefore, Ho3+ ions, which can provide certain red emission besides the green one, are used to the multiply doped system. Figure 9 shows the upconversion emission spectra of Tm3+/Ho3+/Yb3+ triply doped NZPGT glasses (sample V), and four emission bands locate at 477, 545, 657, and 805nm wavelengths. Under low-power pumping, the blue, green, and red emissions are comparable, with increasing the pump power, the blue light intensity exceeds those of green and red ones, due to the three-photon absorption relation on pump power. Therefore, by varying the excitation power, the fluorescence color in sample V changes from yellowish-white to white and bluish-white, as shown in Fig. 10 . The color coordinates (x, y) and CCT are listed in Table 1 , and the tuning characteristic is shown in Fig. 8. The fluorescence color coordinates move along the bottom left direction from the right boundary of the white region, arriving at the pure white region and approaching to the equal energy point (0.333, 0.333) under 236mW laser pump power. The result reveals that the fluorescence color can be efficiently tuned by adjusting the excitation power, and sample V exhibiting bright white light is a preferable candidate for white-lighting optical fiber in achieving illumination at deep and narrow space of human body and developing minimally invasive surgery.
Fig. 9 Upconversion emission spectra of sample V normalized by the blue 477nm peak intensity under 367mW 974nm laser excitation.
Fig. 10 Luminescence photos (a), (b), (c), and (d) present the fluorescences of sample V under 37, 367, 504, and 778mW pump powers, respectively.
Table 1. CIE-1931 color coordinates (x, y) and correlated color temperature (CCT) of the fluorescences from sample V under different excitation powers.
4. Conclusion
In Er3+/Yb3+ codoped Na2O-ZnO-PbO-GeO2-TeO2 (NZPGT) glass fiber, a clear and compact green upconversion ASE trace is observed, indicating the NZPGT glass materials are desirable candidates in fabricating low-phonon energy fiber. Intense green upconversion emission of Er3+, comparable green and red upconversion emissions of Ho3+, and three-photon blue upconversion fluorescence of Tm3+ have been observed. By varying the excitation power of 974nm wavelength laser diode, a series of green and white fluorescences have been obtained in Tm3+/Er3+/Yb3+ and Tm3+/Ho3+/Yb3+ triply doped glass systems, respectively. High-quality green, blue, and white upconversion ASE fluorescences will be produced in RE3+ doped NZPGT glass fibers, and these promising lights can be widely applied for illumination in minimally invasive photodynamic therapy and minimally invasive surgery.
Acknowledgement
This work was supported by National Natural Science Foundation of China (60977014) and Research Grants Council of the Hong Kong Special Administrative Region, China (CityU 1197/08).
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